This invention relates to programmable logic devices, and more particularly to improved macrocells in which the macrocell OR function can be retained during product term allocation and in which product terms can be allocated bidirectionally.
Programmable logic devices, which are capable of being programmed by a user to perform a variety of logic functions, are well known. These devices often are based on a programmable AND array coupled to an OR array. The outputs of the AND array are the logical product of the inputs to the array and are therefore called product terms. The OR logic sums the product terms so that together the two arrays form a complete "sum of products," which can be configured to provide any desired combinatorial logic function. (The outputs of the first array are called product terms regardless of whether the logic array is implemented using the AND-OR configuration or whether another of the equivalent two-level logic configurations is used.)
In some programmable logic devices it is advantageous to partition the OR logic into blocks called macrocells. The macrocells are each connected to a subset of the product terms of the AND array, which, although resulting in some loss of flexibility, generally allows resources to be used more efficiently. Each macrocell typically receives a set number of product terms which can then be summed by the OR logic and, if desired, processed further by additional macrocell logic and temporarily stored in a register within the macrocell. However, because the number of product terms per macrocell is fixed, logic functions that require more product terms cannot be implemented using this approach.
The limitations imposed by having a fixed number of product terms for each macrocell may be overcome by providing circuitry that allows the product term for one macrocell to be allocated to an adjacent macrocell, as shown in commonly-assigned U.S. Pat. No. 5,121,006. For example, if the macrocells in a programmable logic device each contain five product term inputs and in one macrocell six product terms must be summed, the sixth product term can be obtained from a neighboring macrocell. Similarly, if 15 product terms must be summed, three macrocells can be daisychained together.
However, when using the product term allocation scheme described in the above-mentioned U.S. Pat. No. 5,121,006, if only one additional product term input is required, the OR function of the macrocell from which the additional product term is allocated cannot be used for summing the remaining product term inputs in that macrocell. Further, when many macrocells are daisy-chained together, the fixed delay associated with each connection between macrocells may result in an overall delay that is too long. It would therefore be desireable if the magnitude of the cumulative delay could be reduced.